Field of the Invention
The present invention relates to a control device and a stepping motor control method.
Description of the Related Art
There have been proposed various methods for driving a stepping motor which is installed in optical equipment such as an imaging apparatus or the like. The stepping motor readily achieves high resolution with open loop control by a micro step drive which uses a sine wave as a control waveform. Thus, in general, there has been proposed a method for driving a stepping motor with open loop control.
On the other hand, a stepping motor may undesirably be stepped-out upon high velocity rotation. Thus, there has been proposed an advance angle control technique for rotating a stepping motor at high velocity without step-out by providing a rotation position detection mechanism to the stepping motor so as to advance the phase of a control waveform by a predetermined angle.
For example, U.S. Pat. No. 4,963,808 discloses a control technique in which two types of operation modes, i.e., a stepping mode using as a two-phase stepping motor and a closed loop mode using as a DC motor can be switchably used by a control unit. Also, U.S. Pat. No. 6,879,346 discloses a technique in which the maximum velocity corresponding to an advance angle used by closed loop control is measured in advance and then the advance angle is controlled by using the measured data so as to control a motor at any velocity.
However, in the technique for controlling the velocity of a stepping motor at any velocity by advance angle control, responsiveness and linearity upon changing an advance angle may be poor in actual usage. Thus, conventionally, in order to perform closed loop control using an advance angle, an advance angle corresponding to a velocity needs to be individually measured and stored in an apparatus in advance so as to calculate an advance angle corresponding to a target velocity based on the result of measurement by taking into consideration the effect of variance such as motor properties, the mechanism load connected to a motor, or the like.
Here, in an imaging apparatus that performs automatic focusing an imaging sensor, the rotation velocity of a stepping motor for controlling a focus adjusting lens is determined depending on the focal depth of a lens and the frame rate of an imaging sensor. Since the velocity of a stepping motor required for the above control differs depending on the focal distance of a lens, the condition of an object, or the like, the stepping motor needs to be accurately controlled at any velocity. When conventional advance angle control is applied to an apparatus (e.g., imaging apparatus) in which a stepping motor needs to be controlled at any constant velocity, all the advance angle measurement data depending on the drive velocity of the apparatus must be stored in the same in advance. Thus, in an apparatus in which a stepping motor needs to be controlled at any constant velocity, advance angle control is rarely used.